The Dangers of Free Mixing
Shaykhul Islaam Ibnul Qayyim
At-Turuq al-Hikmiyyah p.281
There is no doubt that allowing women into intermingle with men is the basis of calamity and vice. And it is from the greatest causes of the befalling of general punishments. Just as it is the cause of the corruption of both general and specific affairs.
The intermingling of men with women also leads to sin and adultery, and is amongst the causes of demise in general and sexually transmitted diseases. When prostitutes intermingled with Moosa’s soldiers and fornication spread amongst them, Allaah sent down the plague upon them. Seventy-thousand of them died in a single day! The story is well known in the books of Tafseer.
So from the greatest of causes of general demise is the abundance of fornication due to endorsing the intermingling of women with men, and mixing with them whilst beautifying themselves and displaying their adornment. If the rulers knew of the corruption of worldly affairs and the public in this – before the religion – they would have been the severest ones in forbidding intermingling.